Over the last several decades, a variety of diagnostic procedures have been developed for sensing and analyzing activity of the human heart. These include electrocardiography, vectorcardiography and polarcardiography, all of which depend upon related instrumentation used to produce records derived from voltages produced by the heart on the surface of the human body.
The records so produced are graphical in character and require interpretation and analysis to relate the resulting information to the heart condition of the patient or other subject. Historically, such records have been produced directly as visible graphical recordings--wired connections extending from the subject to the recording device. With advances in computer technology, it as become possible to produce such records in the form of digitally stored information for later replication or retrieval and analysis. Likewise, with advances in communication technology, remote (wireless) sensing has become possible.
(a) Electrocardiography
The production of a conventional 12-lead electrocardiogram (ECG) involves the placement of 10 lead electrodes (one of which is a ground or reference electrode) at selected points on the surface of a subject's body. Each electrode acts in combination with one or more other electrodes to detect voltages produced by depolarization and repolarization of individual heart muscle cells. The detected voltages are combined and processed to produce 12 tracings of time varying voltages. The tracings so produced are as follows:
______________________________________ Lead Voltage Lead Voltage ______________________________________ I vL - vR V1 v1 - (vR + vL + vF)/3 II vF - vR V2 v2 - (vR + vL + vF)/3 III vF - vL V3 v3 - (vR + vL + vF)/3 aVR vR - (vL + vF)/2 V4 v4 - (vR + vL + vF)/3 aVL vL - (vR + vF)/2 V5 v5 - (vR + vL + vF)/3 aVF vF - (vL + vR)/2 V6 v6 - (vR + vL + vF)/3 ______________________________________ where, in the standard, most widely used system for making short term electrocardiographic recordings of supine subjects, the potentials indicated above, and their associated electrode positions, are: vL potential of an electrode on the left arm; vR potential of an electrode on the right arm; vF potential of an electrode on the left leg; v1 potential of an electrode on the front chest, right of sternum in the 4th rib interspace; v2 potential of an electrode on the front chest, left of sternum in the 4th rib interspace; v4 potential of an electrode at the left midclavicular line in the 5th ri interspace; v3 potential of an electrode midway between the v2 and v4 electrodes; v6 potential of an electrode at the left midaxillary line in the 5th rib interspace; v5 potential of an electrode midway between the v4 and v5 electrodes; vG (not indicated above) is a ground or reference potential with respect to which potentials vL, vR, vF, and v1 through v6 are measured. Typically though not necessarily, the ground or reference electrode is positioned o the right leg.
Correct interpretation of an ECG requires a great deal of experience since it involves familiarity with a wide range of patterns in the tracings of the various leads. Any ECG which uses an unconventional system of leads necessarily detracts from the body of experience that has developed, in the interpretation of conventional ECGs, and may therefore be considered generally undesirable. The tracings generated would be understandable only by a relative few who were familiar with the unconventional system.
Nevertheless, other leads system have evolved from improvements in instrumentation that have permitted extension of electrocardiography to ambulatory, and even vigorously exercising subjects--and to recordings made over hours, or even days. For example, in stress testing the electrodes are moved from the arms to the trunk, although the same number of electrodes (10) are used. The tracings I, II, III, aVR, aVL and aVF are altered by this modification.
Although a 12-lead ECG is considered to be a cost effective heart test, it is to be noted that the relatively large number of electrodes required play an important role in determining costs--not only in terms of the direct cost of the electrodes themselves, but also terms of the time required to properly position and fix each electrode to a subject's body.
(a) Vectorcardiography
The pattern of potential differences on a body surface resulting from electrical activity of the heart can be mathematically approximated by replacing the heart with a dipole equivalent cardiac generator. The magnitude and orientation of this dipole are represented by the heart vector which is continually changing throughout the cycle of the heart beat. The XYZ coordinates of the heart give rise to time varying xyz signals, which may be written out as xyz tracings. Orthogonal leads to give these tracings were developed by Ernest Frank (see An Accurate, Clinically Practical System For Spatial Vectorcardiography, Circulation 13: 737, May 1956). Frank experimentally determined the image surface for one individual, and from this proposed a system using seven electrodes on the body, plus a grounding electrode. The conventional letter designations for such electrodes, and their respective positions were:
E at the front midline; PA1 M at the back midline; PA1 I at the right mid-axillary line; PA1 A at the left mid-axillary line; PA1 C at a 45.degree. angle between the front midline and the left mid-axillary line; PA1 F on the left leg; PA1 H on the back of the neck.
The first five electrodes (E, M, I, A and C) were all located at the same transverse level--approximately at the fourth of the fifth rib interspace. A linear combining network of resistors attached to these electrodes gave suitably scaled x, y and z voltaage signals as outputs.
Unfortunately, xyz tracings are not as easy to interpret as 12 lead ECGs. However, Frank intended his system for a different purpose: vectorcardiography.
Vectorcardiography abandons the horizontal time coordinate of the ECG in favour of plots or tracings of the orientation and magnitude of the heart vector on each of three planes: a frontal (xy) plane plotting an x-axis (right arm to left arm) against a y-axis (heat to foot); a transverse (xz) plane plotting the x-axis against a z-axis (front to back), and a sagittal plane plotting the y-axis against the z-axis.
Although it has long formed a basis for teaching electrocardiography, vectorcardiography has never become widely used. The technique was demanding and the system of electrode placement was different from that required for the ECG. Extra work was required, and it would still be necessary to record a 12-lead ECG separately with a different placement of electrodes.
An alternative to the Frank lead sysftem that required only four active electrodes (R(right arm), A, F, E), and that used a resistor network based on Frank's image surface data was proposed in 1958 by G. E. Dower (the inventor herein) and J. A. Osborne (see A Clinical Comparison of Three VCG Lead Systems Using Resistance-Combining Networks, Am Heart J 55: 523 1958). However, the xyz signals produced were sometimes different from those of Frank's lead system, and the RAFE system was not adopted.
(c) Polarcardiography
An alternative representation of the heart vector, known as polarcardiography, has been exploited since the early 1960's (see G. E. Dower, Polarcardiography, Springfield, Ill, Thomas, 1971). It has certain inherent advantages in defining abnormalities, and forms the basis of a successful program for automated analysis. Based on xyz signals, polarcardiography employs the Frank lead system. In order to render it competitive with the established 12-lead ECG, the lead vector concept has been employed to derive a resistor network than would linearly transform the xyz signals into analogs of the 12-lead ECG signals (see G. E. Dower, A Lead Synthesizer for the Frank Lead System to Simulate the Standard 12-Lead Electrocardiogram, J. Electrocardiol 1: 101, 1968; G. E. Dower, H. B. Machado, J. A. Osborne, On Deriving the Electrocardiogram From Vectorcardiographic Leads, Clin Cardiol 3: 97, 1980; and G. E. Dower, The ECGD: A Derivation of the ECG from VCG leads (editorial), J. Electrocardiol 17: 189, 1984). The ECG thus [d]erived is commonly referred to as the ECGD. Because the ECGD can be acceptable to an interpreting physician, it is not necessary for the technician to apply the electrodes required for a conventional ECG. Further, associated computer facilities can make vectorcardiograms and other useful displays available from the xyz recordings. Nevertheless, the number of electrodes called for by the Frank lead system are required. In addition, the effort required by the technician recording the xyz signals is about the same as for a conventional ECG.
The primary object of the present invention is to provide a method and apparatus for sensing and analyzing activity of the human heart, and which requires a reduced number of electrodes to produce accurate simulations of conventional 12-lead electrocardiograms and vectorcardiograms.